Dynamic Evolution of Functional Groups and Thermal Oxidation Effects in Acid/alkali-Eroded Coal During Heating Processes

Coal mining has many serious hazards, and the spontaneous combustion phenomenon is one of them. Many factors can influence the tendency for occurrence of this phenomenon. The maceral content is one of the coal inherent characteristics, which has a crucial aspect in the occurrence of this process. Until now, this parameter has not been discussed in a comprehensive study by the scientists. Thus a final conclusion about the effect of maceral content on spontaneous coal combustion is required. Maceral content is directly affected by the growth of coalification and origin of coal formation. This study aimed at developing a new statistical model for predicting the spontaneous coal combustion tendency based on the maceral content. In this work, firstly, 51 coal samples with a variety of maceral contents were collected for training data of the overall underground coal fields in Iran, and the Crossing Point Temperature (CPT) test was carried out on each coal sample. Then the method of regression analysis was used for modeling and predicting the spontaneous coal combustion tendency. For validation and testing, 11 coal samples of another coalfield were collected, the CPT test was carried out on each one of them, and the results obtained were compared using the regression equation. Validation of the developed model was checked by the F test. By examining the experimental results and comparing the CPT test values, it was found that with increase in the vitrinite and liptinite contents and decrease in the inertinite content, the coal samples displayed a higher tendency for spontaneous combustion. The results also showed that the spontaneous combustion tendency could be reliably predicted using the proposed model.

Coal plays a crucial role in the energy structure of China. However, coal oxidation in goaf areas can lead to spontaneous combustion and the release of harmful substances, such as CO2 and As2O3, which pose environmental hazards. The transport patterns of CO2 and As2O3 from the mining area to the surface were investigated, and the environmental impacts of their transport were preliminarily investigated. This study aimed to obtain data on the relationship between the release of CO2 and As2O3 during spontaneous coal combustion and coal temperature, serving as baseline information for numerical models of spontaneous coal combustion. From a macro perspective, we explored the correlation between the simultaneous transport of CO2 and As2O3. Additionally, we constructed a multi-physics coupled model of spontaneous coal combustion in goaf areas using COMSOL software to analyze the transport patterns of CO2 and As2O3 released during spontaneous coal combustion in the three-zone overburden and their collaborative transport. The results indicated consistent release patterns of CO2 and As2O3, with CO2 released in much larger quantities than As2O3, thus providing conditions for CO2 to act as a carrier for As2O3 transport. During independent transport in the three-zone overburden, CO2 was transported to the surface, whereas a significant portion of As2O3 was enriched in the fracture zone. However, when CO2 and As2O3 were transported collaboratively, As2O3 reached the surface and the transport patterns of both substances in the overburden remained consistent, indicating that CO2 facilitated the transport of As2O3. This study provides an important theoretical basis for the remediation of spontaneous coal combustion in goaf areas, contributing to the effective control of CO2 and As2O3 emissions and the protection of mining environments. The surface concentration of CO2 peaked at 1560 mol/m3 at 45 d, and the surface concentration of As2O3 also peaked at 0.016 mol/m3.

Spontaneous combustion of coal is a complex underground mining disaster, which mainly threats mine safety and efficiency. Several factors usually cause spontaneous combustion of coal, such as gas concentration, ventilation and coal properties. In this study, spontaneous combustion tendencies of coal mines were predicted considering the effective parameters for an underground coal mine in Turkey. Multinomial logistic regression, a multivariate statistical technique, was applied. Gas concentrations (CH4, CO, O2) and air velocity were defined as factors affecting spontaneous coal combustion. Fire hazard levels of the coal mines were determined as ‘normal situation’ and ‘potential combustion’. It was observed that CH4 and CO variables and CH4 × CO interaction were effective in the formation of clusters. The results indicate that Mine I is more liable to spontaneous combustion than Mine II and Mine III. At the same time, the effects of variations in factors are examined in the study.

One of the most catastrophic events that can occur during coal mining is a fire triggered by spontaneous combustion of the coal. Due to complex geological conditions, some coal mines in China have the characteristics of shallow burial and small spacing between coal seams. With the increase of mining depth, there will be many cracks in the overlying goaf. Surface water and groundwater flow into the goaf along these fissures. After long-term water immersion, the increase of ground temperature will lead to the spontaneous combustion of coal. A thorough analysis of the impact of water immersion on the spontaneous combustion of coal necessitates a review of the most recent findings in this field. The review will promote the further development of the theory and prevention of spontaneous combustion of water-immersed coal and provide theoretical help for monitoring and preventing spontaneous combustion of water-immersed coal. The creation of water-immersed coal and the reason for spontaneous combustion is investigated, as are the characteristics of the water accumulation in the goaf and their origins. The research status of spontaneous combustion of water-immersed coal is presented and illustrated domestically and internationally. On this basis, the theoretical and experimental research directions, problems, and prospects of spontaneous combustion of water-immersed coal are put forward. This study can provide a theoretical basis for monitoring, preventing, and controlling the spontaneous combustion of water-immersed coal, and provide a reference for reducing energy waste.

The spontaneous combustion of coals is a natural phenomenon that causes serious losses in the mining sector as well as environmental problems as a result of greenhouse gases released during spontaneous combustion of coal. In this study, for the first time, the effect of coal reactor size on the spontaneous combustion tendency parameters (crossing point temperature (CPT), average heating rate (AHR), and liability index (IFCC)) of the coal samples having two different ranks (bituminous coal and lignite) have been searched by the use of crossing point method. According to the results obtained from the reactors manufactured in three different sizes (R-1, R-2, and R-3), ignition temperatures of coal samples were 161–172 °C for bituminous coal and 143–175 °C for lignite; average temperature rise was 0.92–1.45 °C min−1 for bituminous coal and 1.47–2.98 °C min−1 for lignite; spontaneous combustion risk index was determined in the range of 5.61–8.88 min−1 for bituminous coal and 8.39–20.79 min−1 for lignite. As the reactor size decreased, spontaneous combustion tendencies of lignite samples increased, and no significant change was observed in bituminous coal samples. It has been found that the effect of reactor size on the spontaneous combustion of coal gives significant results in low-rank coals but has no effect on high-rank coals.

Characteristics of polycyclic aromatic hydrocarbon release during spontaneous combustion of coal and gangue in the same coal seam

Oxidation and spontaneous combustion characteristics of particulate coal under stress–heat–gas interactions

Free radicals and metal ions are key factors causing coal spontaneous combustion. In order to effectively suppress coal spontaneous combustion, experiments were conducted using luteolin as an inhibitor, luteolin is abbreviated as LUT. The influence of different concentrations of LUT on the spontaneous combustion heat characteristics of coal was tested using a C600 high-precision microcalorimeter, and the inhibition mechanism was inferred by FTIR. The experimental results show that LUT is an effective coal oxygen inhibitor, under the same action conditions, different concentrations of LUT can absorb heat to different degrees, delay the initial exothermic temperature of coal, prolong the heat absorption phase of coal. Compared with raw coal, 6 wt% LUT absorbs the most heat, the starting exothermic temperature is delayed by 46°C, and the activation energy is increased by 47.89 kJ/mol, the functional group -OH content decreased by 43.4%, -CH3 decreased by 32.1%, -CH2-decreased by 37.76%. Based on this, the mechanism of inhibition is inferred, LUT not only removes free radicals (ROO•) during the chain initiation stage and captures free radicals (•OH) in the reaction chain but also chelates metal ions to inhibit the catalytic effect of metal ions during the free radical reaction, which can play a dual role in preventing free radical generation and interrupting the free radical chain reaction. The results of the study can provide a novel, environmentally friendly and effective way to prevent the spontaneous combustion of coal.

New insight into proactive goaf inertisation for spontaneous combustion management and control

Unique spatial methane distribution caused by spontaneous coal combustion in coal mine goafs: An experimental study

This paper examines the relationship between the advancement of the working face and the distribution of spontaneous coal combustion under a low air leakage wind speed in a goaf. Based on the unsteady-state simulation method, the process of spontaneous coal combustion was simulated in a fully mechanized coal mining face at different advancing speeds. The relationship between different advancing speeds and the distribution area of the oxidation zone of spontaneous coal combustion in a goaf was clarified. A safe advancing speed was determined. Furthermore, the advancement of the working face altered the area of spontaneous high temperature inside the goaf. An increase in the advancing speed increased the length of time of spontaneous combustion in the leftover coal. An exponential relationship was demonstrated between the spontaneous combustion area and the advancement of the fully mechanized mining face. When the oxygen concentration was used as a calculation indicator, the width of the oxidation zone was asymmetrical on the inlet and outlet, and the advancing distance of the working surface increased when the distance was shorter than 90 m and the return side was smaller than the inlet side. When the advancing distance was longer than 90 m, the width of the spontaneous combustion oxidation zone distribution remained unchanged. The study clarified the relationship between the distribution of the danger area for spontaneous coal combustion and advancing speed. This could provide a theoretical basis for the prevention and control technology of the spontaneous combustion of broken coal in a dynamic goaf.

Understanding the evolution of air flow fields and leakage patterns in large-scale goaf areas is crucial for preventing spontaneous combustion of residual coal and ensuring mining safety. Current understanding of air leakage evolution in interconnected goaf areas remains insufficient for effective hazard control. This study aims to investigate the characteristics of air flow fields and establish prevention strategies for large-scale goaf areas. Fast Lagrangian analysis of continua in 3 dimensions (FLAC3D) numerical simulations were employed to analyze the fracturing and stress distribution of overlying strata, establishing a three-dimensional porosity model. The spatiotemporal evolution of air leakage flow fields was then simulated using FLUENT computational fluid dynamics software. Results indicated that stress distribution in the large goaf area was concentrated in the central region, exhibiting a “saddle-shaped” fracture pattern with a maximum stress of 35.4 MPa. The void ratio distribution displayed an elliptical pattern, with the lowest value of 0.097 in the compacted central goaf area, increasing to 0.34 at the boundary. Measurements of the air leakage field revealed inlet and outlet flow rates of 242.63 and 232.62 m3/min, respectively. The coupled hazardous zones were primarily concentrated around air leakage sources and their adjacent goaf areas, extending 336.42 m horizontally and 224.21 m vertically, encompassing a total area of 17 433.04 m2. Based on these findings, a three-zone prevention strategy was developed, providing new insights into spontaneous combustion control for large-scale goaf areas.

Spontaneous combustion of coal is one of the main natural disasters in the process of coal mining, and spontaneous combustion of sulfur-containing coal is an urgent problem to be solved. In order to understand the influence of different sulfur structures and sulfur content on spontaneous combustion tendency of coal, the self-developed synchronous temperature compensation adiabatic oxidation experiment system was used to study the oxidation self-heating performance of a series of sulfur-containing “coal-like materials.” Through the analysis of the heating rate of coal containing specific sulfur structure, it is considered that the time of adiabatic oxidation temperature of different organic sulfur “coal” to rise to 70°C is dimethyl sulfoxide > phenylene sulfide > dibenzothiophene. By analyzing the oxidation temperature rise characteristics of organic sulfur models with different sulfur contents, it is believed that the higher sulfur content, the faster the low temperature oxidation reaction of “coal-like substance,” the faster the temperature rise rate, and the shorter the time to reach the uncontrollable self-heating temperature, so the shorter the spontaneous combustion period. In general, in the stage of low temperature oxidation, active sulfur containing small molecules (dimethyl sulfoxide) promote the oxidation temperature rise, stable structure, larger molecular weight of dibenzothiophene slow down the “coal-like” oxidation temperature rise.

Determining a reasonable barrier pillar along gob-side entry of spontaneous combustion coal seam is of great significance to the prevention of spontaneous combustion. In this paper, considering the actual situation of 4301 and 4302 working faces of II-class spontaneous combustion coal seam in Changheng Mine, the characteristics of rock mass collapse and mining-induced stress redistribution of barrier pillar and adjacent area were analyzed, the cusp catastrophe model of coal pillar instability and the theoretical model of limit width of coal pillar air leakage spontaneous combustion were established, and a measurement scheme of side abutment pressure in 4301 working face was carried out. The theoretical model of the coal pillar along the gob-side entry shows that its instability is related to density of overlying strata, physical and mechanical parameters of coal body, dip angle of coal seam and weak face, and buried depth and mining width of working face. And the coal pillar in 4302 working face will be unstable if the width a ≤ 6.8 m . Field measured data shows that 2-8 m is the stress-relaxation area of side solid coal. Then, the width of coal pillar was determined as 7 m. The maximum displacement of rib-to-rib and roof-to-floor of 4302 tailentry was 520 mm and 280 mm, respectively. The coal spontaneous combustion (CSC) observation monitoring results showed that the carbon monoxide volume fraction was in the normal range during the entire process of mining, and no other signs of gas were found. The research results can provide a reference for the barrier pillar designing in the mines under similar conditions.

Experimentalstudy on microscopic action of different form moistureon coal spontaneous combustion